How do you properly use namespaces in C

Question

I come from a Java background  where packages are used  not namespaces  I m used to putting classes that work together to form a complete object into packages  and then reusing them later from that package  But now I m working in C     How do you use namespaces in C    Do you create a single namespace for the entire application  or do you create namespaces for the major components  If so  how do you create objects from classes in other namespaces

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Bigger C   projects I ve seen hardly used more than one namespace  e g  boost library     Actually boost uses tons of namespaces  typically every part of boost has its own namespace for the inner workings and then may put only the public interface in the top-level namespace boost   Personally I think that the larger a code-base becomes  the more important namespaces become  even within a single application  or library   At work we put each module of our application in its own namespace   Another use  no pun intended  of namespaces that I use a lot is the anonymous namespace   namespace     const int CONSTANT   42      This is basically the same as   static const int CONSTANT   42    Using an anonymous namespace  instead of static  is however the recommended way for code and data to be visible only within the current compilation unit in C

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Also  note that you can add to a namespace  This is clearer with an example  what I mean is that you can have   namespace MyNamespace       double square double x    return x   x        in a file square h  and  namespace MyNamespace       double cube double x    return x   x   x        in a file cube h  This defines a single namespace MyNamespace  that is  you can define a single namespace across multiple files

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Generally speaking  I create a namespace for a body of code if I believe there might possibly be function or type name conflicts with other libraries  It also helps to brand code  ala boost

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Vincent Robert is right in his comment How do you properly use namespaces in C     Using namespace Namespaces are used at the very least to help avoid name collision  In Java  this is enforced through the  quot org domain quot  idiom  because it is supposed one won t use anything else than his her own domain name   In C    you could give a namespace to all the code in your module  For example  for a module MyModule dll  you could give its code the namespace MyModule  I ve see elsewhere someone using MyCompany  MyProject  MyModule  I guess this is overkill  but all in all  it seems correct to me  Using  quot using quot  Using should be used with great care because it effectively import one  or all  symbols from a namespace into your current namespace  This is evil to do it in a header file because your header will pollute every source including it  it reminds me of macros      and even in a source file  bad style outside a function scope because it will import at global scope the symbols from the namespace  The most secure way to use  quot using quot  is to  import select symbols  void doSomething        using std  string      string is now  quot imported quot   at least                            until the end of the function    string a  quot Hello World  quot        std  cout  lt  lt  a  lt  lt  std  endl      void doSomethingElse        using namespace std      everything from std is now  quot imported quot   at least                            until the end of the function    string a  quot Hello World  quot        cout  lt  lt  a  lt  lt  endl      You ll see a lot of  quot using namespace std   quot  in tutorial or example codes  The reason is to reduce the number of symbols to make the reading easier  not because it is a good idea   quot using namespace std   quot  is discouraged by Scott Meyers  I don t remember exactly which book  but I can find it if necessary   Namespace Composition Namespaces are more than packages  Another example can be found in Bjarne Stroustrup s  quot The C   Programming Language quot   In the  quot Special Edition quot   at 8 2 8 Namespace Composition  he describes how you can merge two namespaces AAA and BBB into another one called CCC  Thus CCC becomes an alias for both AAA and BBB  namespace AAA      void doSomething        namespace BBB      void doSomethingElse        namespace CCC      using namespace AAA      using namespace BBB      void doSomethingAgain        CCC  doSomething        CCC  doSomethingElse        You could even import select symbols from different namespaces  to build your own custom namespace interface  I have yet to find a practical use of this  but in theory  it is cool

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To avoid saying everything Mark Ingram already said a little tip for using namespaces   Avoid the  using namespace  directive in header files - this opens the namespace for all parts of the program which import this header file  In implementation files    cpp  this is normally no big problem - altough I prefer to use the  using namespace  directive on the function level   I think namespaces are mostly used to avoid naming conflicts - not necessarily to organize your code structure  I d organize C   programs mainly with header files   the file structure   Sometimes namespaces are used in bigger C   projects to hide implementation details   Additional note to the using directive  Some people prefer using  using  just for single elements       using std  cout    using std  endl

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I ve used C   namespaces the same way I do in C   Perl  etc   It s just a semantic separation of symbols between standard library stuff  third party stuff  and my own code   I would place my own app in one namespace  then a reusable library component in another namespace for separation

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Note that a namespace in C   really is just a name space   They don t provide any of the encapsulation that packages do in Java  so you probably won t use them as much

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marius  Yes  you can use several namespaces at a time  eg   using namespace boost     using namespace std     shared ptr lt int gt  p new int 1         shared ptr belongs to boost    cout  lt  lt   cout belongs to std     lt  lt  endl       cout and endl are in std    Feb  2014 --  Has it really been that long    This particular example is now ambiguous  as Joey points out below  Boost and std   now each have a shared ptr

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Another difference between java and C    is that in C    the namespace hierarchy does not need to mach the filesystem layout   So I tend to put an entire reusable library in a single namespace  and subsystems within the library in subdirectories    include  lib module1 h   include  lib module2 h   lib  class1  v   new lib  class1      I would only put the subsystems in nested namespaces if there was a possibility of a name conflict

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I did not see any mention of it in the other answers  so here are my 2 Canadian cents   On the  using namespace  topic  a useful statement is the namespace alias  allowing you to  rename  a namespace  normally to give it a shorter name  For example  instead of   Some  Impossibly  Annoyingly  Long Name  For  Namespace  Finally  TheClassName foo  Some  Impossibly  Annoyingly  Long Name  For  Namespace  Finally  AnotherClassName bar    you can write   namespace Shorter   Some  Impossibly  Annoyingly  Long Name  For  Namespace  Finally  Shorter  TheClassName foo  Shorter  AnotherClassName bar

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Don t listen to every people telling you that namespaces are just name-spaces   They are important because they are considered by the compiler to apply the interface principle  Basically  it can be explained by an example   namespace ns    class A       void print A a           If you wanted to print an A object  the code would be this one   ns  A a  print a     Note that we didn t explicitly mention the namespace when calling the function  This is the interface principle  C   consider a function taking a type as an argument as being part of the interface for that type  so no need to specify the namespace because the parameter already implied the namespace   Now why this principle is important  Imagine that the class A author did not provide a print   function for this class  You will have to provide one yourself  As you are a good programmer  you will define this function in your own namespace  or maybe in the global namespace   namespace ns    class A          void print A a        And your code can start calling the print a  function wherever you want  Now imagine that years later  the author decides to provide a print   function  better than yours because he knows the internals of his class and can make a better version than yours   Then C   authors decided that his version of the print   function should be used instead of the one provided in another namespace  to respect the interface principle  And that this  upgrade  of the print   function should be as easy as possible  which means that you won t have to change every call to the print   function  That s why  interface functions   function in the same namespace as a class  can be called without specifying the namespace in C     And that s why you should consider a C   namespace as an  interface  when you use one and keep in mind the interface principle   If you want better explanation of this behavior  you can refer to the book Exceptional C   from Herb Sutter

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You can also contain  using namespace      inside a function for example   void test const std  string amp  s        using namespace std      cout  lt  lt  s

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Namespaces are packages essentially  They can be used like this   namespace MyNamespace     class MyClass              Then in code   MyNamespace  MyClass  pClass   new MyNamespace  MyClass      Or  if you want to always use a specific namespace  you can do this   using namespace MyNamespace   MyClass  pClass   new MyClass      Edit  Following what bernhardrusch has said  I tend not to use the  using namespace x  syntax at all  I usually explicitly specify the namespace when instantiating my objects  i e  the first example I showed    And as you asked below  you can use as many namespaces as you like

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In Java   package somepackage  class SomeClass      In C     namespace somenamespace       class SomeClass        And using them  Java   import somepackage    And C     using namespace somenamespace    Also  full names are  somepackge SomeClass  for Java and  somenamespace  SomeClass  for C    Using those conventions  you can organize like you are used to in Java  including making matching folder names for namespaces  The folder- package and file- class requirements aren t there though  so you can name your folders and classes independently off packages and namespaces

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I prefer using a top-level namespace for the application and sub namespaces for the components   The way you can use classes from other namespaces is surprisingly very similar to the way in java  You can either use  use NAMESPACE  which is similar to an  import PACKAGE  statement  e g  use std  Or you specify the package as prefix of the class separated with       e g  std  string  This is similar to  java lang String  in Java

[c++] How do you properly use namespaces in C++?

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